The present invention relates to a flexible board that includes a film having flexibility and to a connection method and a connection structure of the board.
As a flexible board for connecting, for example, an LCD panel with a main body circuit board of a portable telephone, there has conventionally been one as shown in FIG. 6.
The flexible board is provided with an LCD driver chip 110 for driving an LCD panel 115, the chip being mounted on a surface of a film 101 by a COF (chip on film) method. A sealing resin 112 is provided so as to bury a gap between the LCD driver chip 110 and the film 101. The LCD driver chip 110 is connected to an input outer lead 106 and an output outer lead 111 via inner leads 113 formed on the film 101. The input outer lead 106 is connected to a main body circuit board 102, while the output outer lead 111 is connected to the LCD panel 115. A solder resist 109 is provided on the surface of the film 101 so as to cover the input outer lead 106 and output outer lead 111 in the peripheral portions of the LCD driver chip 110.
Electroless plating of Sn (tin) is provided on the surface of the inner leads 113, and the LCD driver chip 110 and the inner leads 113 are connected together with an Au—Sn eutectic alloy formed by thermocompression bonding of Sn to an Au protruding electrode 114 of the LCD driver chip 110.
The output outer lead 111 is connected to a terminal electrode of the LCD panel 115 with an anisotropic conductive adhesive 116 in which metal particles are diffused in the resin in a film-like form. The metal particles of the anisotropic conductive adhesive 116 have a diameter of about 5 μm. By carrying out the thermocompression bonding of the output outer lead 111 of the flexible board to the LCD panel 115 with interposition of the anisotropic conductive adhesive 116, continuity between the output outer lead 111 and the terminal electrode is obtained, and insulation between the film 101 and the LCD panel 115 is secured.
The input outer lead 106 is connected to an electrode terminal 107 inside a connector 103 provided for the main body circuit board 102. The connector 103 has a slit 104 of a roughly bracket-shaped sectional configuration, and an end portion of the film 101 is inserted in the slit 104. Then, the input outer lead 106 is pressurized against the electrode terminal 107 from the lower surface side of the film 101 by means of a stopper 105 provided in the slit 104, electrically connecting the input outer lead 106 to the electrode terminal 107 and fixing the film 101 to the connector 103. The connector 103 of the type is called the ZIF (Zero Insertion Force) connector.
The film 101 normally used for the flexible board has a thickness much thinner than the dimension of the slit 104 of the connector (the thickness is, for example, 103 μm when UPILEX produced by Ube Industries, Ltd. is used), and therefore, the stopper 105 cannot fix the single body of the film 101. Therefore, a reinforcing plate 120 of a prescribed thickness is bonded to a surface opposite from the surface on which the input outer lead 106 is provided in the portion inserted in the slit 104 of the film 101. By pressurizing the reinforcing plate 120 by means of the stopper 105, the film 101 is firmly connected to the connector 103. Moreover, the mechanical strength of the flexible board is increased by the reinforcing plate 120, making it difficult to damage the flexible board when the board is thrust into or pulled out of the connector 103. With regard to the reinforcing plate of the kind, there is one disclosed in, for example, JP 2001-119120 A.
The reinforcing plate 120 is constructed of a PI (polyimide) piece and a thermosetting adhesive for bonding the PI piece to the film 101. The process of forming the reinforcing plate 120 on the film 101 is constituted of a thermocompression bonding process for temporarily bonding the PI piece onto the film 101 and a curing process for completely hardening the thermosetting adhesive.
The thickness of the flexible board in the portion inserted in the slit 104 of the connector is about 300 μm, which includes 175 μm of the PI piece, 35 μm of the thermosetting adhesive and 103 μm of the film 101. The thickness is required to be maintained without change from when the reinforcing plate 120 is formed. However, the thermosetting adhesive is roughly completely hardened by the curing, and therefore, the thickness of the adhesive is scarcely reduced with a lapse of time. Therefore, the reduction in the thickness of the reinforcing plate 120 is effectively prevented, and the flexible board is stably fixed to the connector 103.
As described above, by connecting the input outer lead 106 to the main body circuit board 102 by the connector 103 of simple insertion and withdrawal, labor saving can be achieved further than when the connection is achieved by ACF (anisotropic conductive film) or soldering in the assembling process of the portable telephone.
The conventional flexible board normally has an elongate form of the material of the film 101 in the manufacturing processes, and the length sometimes reaches about 40 m despite that the width of the material is about 70 mm. Therefore, in the process of mounting the LCD driver chip 110 or other processes, the material of the film 101 wound around a reel is to be fed to a chip mounting apparatus in a reel-to-reel system. Moreover, the formation process of the reinforcing plate 120 also needs to be carried out on the material of the film 101 on the reel. Therefore, an apparatus for continuously carrying out the bonding of the PI piece to the material of the film 101 or the like in a reel-to-reel manner is needed. Furthermore, an apparatus of a curing furnace, an oven or the like for completely hardening the thermosetting adhesive of the reinforcing plate 120 is needed.
As described above, the conventional flexible board, which needs the comparatively expensive apparatus for the formation of the reinforcing plate 120, therefore has a problem that the manufacturing cost is comparatively high and a further increase in the cost is caused by the running cost of the apparatus and the addition of an inspection process.